This invention is generally directed to improvements in semiconductor memories, and particularly to a redundancy system for use in high speed, wide-word semiconductor memories.
It is well known in the art that the yields of semiconductor memories can be improved by including redundant memory cells within each memory. See, for example, U.S. Pat. No. 4,250,570. When the manufacturer's probe test has identified one or more defective cells in the main memory array, the redundant cells are programmed to respond to the addresses of the defective cells.
Among some of the problems redundancy can present to the designer is the compromise between performance and yield. As those skilled in the art are aware, certain types of redundancy schemes, particularly for very high speed, wide-word memories, can detract from the memory's access time. In addition, certain types of redundancy schemes do not provide their expected increases in yield. Accordingly, there is a need for an improved redundancy system which provides significant improvements in yield without detracting from performance, especially for memories with very high speed (45 nanosecond) access times.
Another problem with certain redundancy schemes is their potential for causing on-chip fuses to be inadvertently blown during memory power up. Such fuses are part of the redundancy system and are typically blown or left unblown during probe test to program redundant memory cells. Those fuses which are intended to be unblown can be inadvertently blown during power up by transient currents developed, for example, by the charging of parasitic capacitances. The inadvertent blowing of a fuse is clearly undesirable since it erroneously programs one or more redundant memory cells.
Accordingly, it is generally an object of the invention to provide an improved redundancy system for high speed semiconductor memories.
It is a more specific object of the invention to provide a redundancy system which is particularly adapted for use with a wide-word, high speed memory, and which provides significant improvements in yields without impairing the memory's performance.
It is another object of the invention to provide such a redundancy system which protects programming fuses from being inadvertently blown.